A Birthday Cake

Ingredients

• 500g icing sugar
• 510g caster sugar
• 625g unsalted butter, at room temperature
• 0.75 tsp fine salt
• 1 tbsp baking powder
• For the Cake:
• 6 large egg whites, room temperature
• 250ml whole milk, at room temperature
• 450g unsalted butter, at room temperature, plus more for preparing the tins
• 420g self-raising flour, sifted, plus more for preparing the tins
• 1.5 tsp vanilla essence, at room temperature
• About 1.5kg vanilla/chocolate icing
• Food colouring (optional)
• Enough candles to equal the age of the birthday celebrant
• For the Icing:
• 125ml plus 15ml whole milk, at room temperature
• 1 tbsp plus 0.75 tsp vanilla essence, at room temperature
• 25ml freshly squeezed lemon juice
• 3g finely grated lemon zest
• 250g chocolate, melted (for chocolate icing)